1. Field of the Invention
This invention relates generally to the field of computer systems. More particularly, the invention relates to an apparatus and method for implementing a multi-level memory hierarchy.
2. Description of the Related Art
A. Current Memory and Storage Configurations
One of the limiting factors for computer innovation today is memory and storage technology. In conventional computer systems, system memory (also known as main memory, primary memory, executable memory) is typically implemented by dynamic random access memory (DRAM). DRAM-based memory consumes power even when no memory reads or writes occur because it must constantly recharge internal capacitors. DRAM-based memory is volatile, which means data stored in DRAM memory is lost once the power is removed. Conventional computer systems also rely on multiple levels of caching to improve performance. A cache is a high speed memory positioned between the processor and system memory to service memory access requests faster than they could be serviced from system memory. Such caches are typically implemented with static random access memory (SRAM). Cache management protocols may be used to ensure that the most frequently accessed data and instructions are stored within one of the levels of cache, thereby reducing the number of memory access transactions and improving performance.
With respect to mass storage (also known as secondary storage or disk storage), conventional mass storage devices typically include magnetic media (e.g., hard disk drives), optical media (e.g., compact disc (CD) drive, digital versatile disc (DVD), etc.), holographic media, and/or mass-storage flash memory (e.g., solid state drives (SSDs), removable flash drives, etc.). Generally, these storage devices are considered Input/Output (I/O) devices because they are accessed by the processor through various I/O adapters that implement various I/O protocols. These I/O adapters and I/O protocols consume a significant amount of power and can have a significant impact on the die area and the form factor of the platform. Portable or mobile devices (e.g., laptops, netbooks, tablet computers, personal digital assistant (PDAs), portable media players, portable gaming devices, digital cameras, mobile phones, smartphones, feature phones, etc.) that have limited battery life when not connected to a permanent power supply may include removable mass storage devices (e.g., Embedded Multimedia Card (eMMC), Secure Digital (SD) card) that are typically coupled to the processor via low-power interconnects and I/O controllers in order to meet active and idle power budgets.
With respect to firmware memory (such as boot memory (also known as BIOS flash)), a conventional computer system typically uses flash memory devices to store persistent system information that is read often but seldom (or never) written to. For example, the initial instructions executed by a processor to initialize key system components during a boot process (Basic Input and Output System (BIOS) images) are typically stored in a flash memory device. Flash memory devices that are currently available in the market generally have limited speed (e.g., 50 MHz). This speed is further reduced by the overhead for read protocols (e.g., 2.5 MHz). In order to speed up the BIOS execution speed, conventional processors generally cache a portion of BIOS code during the Pre-Extensible Firmware Interface (PEI) phase of the boot process. The size of the processor cache places a restriction on the size of the BIOS code used in the PEI phase (also known as the “PEI BIOS code”).
B. Phase-Change Memory (PCM) and Related Technologies
Phase-change memory (PCM), also sometimes referred to as phase change random access memory (PRAM or PCRAM), PCME, Ovonic Unified Memory, or Chalcogenide RAM (C-RAM), is a type of non-volatile computer memory which exploits the unique behavior of chalcogenide glass. As a result of heat produced by the passage of an electric current, chalcogenide glass can be switched between two states: crystalline and amorphous. Recent versions of PCM can achieve two additional distinct states.
PCM provides higher performance than flash because the memory element of PCM can be switched more quickly, writing (changing individual bits to either 1 or 0) can be done without the need to first erase an entire block of cells, and degradation from writes is slower (a PCM device may survive approximately 100 million write cycles; PCM degradation is due to thermal expansion during programming, metal (and other material) migration, and other mechanisms).